Semiconductor integrated circuit chips are composed typically of three different classes of materials, semiconductors (typically silicon), insulators and metals. The most common method of depositing the metal layer is by sputtering, which is also called physical vapor deposition (PVD).
Most commercial sputtering is performed in DC magnetron plasma reactors, such as the Endura 5500 PVD Reactor, available from Applied Materials, Inc. of Santa Clara, Calif., although RF magnetron sputtering is also known. Ohm discloses many of the features of a conventional PVD reactor in U.S. Pat. No. 4,874,494. A wafer or other substrate is supported on a pedestal electrode in close opposition to a target electrode composed of at least part of the material to be sputter deposited. The reactor chamber is filled with argon and a negative DC bias is applied to the target electrode with respect to the pedestal electrode. The bias is sufficiently high to cause the argon to be excited to a plasma state. The resultant positively charged argon ions are attracted to the target and strike it with sufficient energy to dislodge atomic sized clusters of target atoms from the target. That is the target is sputtered. The sputtered particles travel ballistically across the chamber, and some of them strike and stick to the substrate, to thereby sputter deposit the target material on the substrate.
In the case of aluminum or titanium sputtering, the target electrode is composed of these conductive materials. In reactive sputtering of a compound material, the target contains only some of the material components, and the chamber is filled with a gas composed of the remaining material components. The sputtered particles chemically react with the gas and together they deposit on the compound as the compound. The most widely used reactively sputtered compound material is TiN, which is formed from a titanium sputtering target and nitrogen gas.
Most commercial sputter reactors rely on magnetron sputtering to increase the sputtering rate. In a magnetron sputter reactor, a magnet is positioned closely adjacent to the back of the target so that its magnetic field extends into the plasma region adjacent to the front of the target. The magnetic field traps a substantial density of plasma electrons in this area, and to preserve charge neutrality inside the quasi-neutral plasma body the concentration of argon ions also increases. As a result, the target sputtering rate is significantly increased.
Most magnetron magnets are formed as an array of small permanent horseshoe magnets producing a magnetic field in the plasma generally parallel to the target face. One method of producing fairly uniform sputtering for a circular target is to arrange the horseshoe magnets in the outline of a kidney shape and then to rotate the array as a whole about the center of the target. That is, the magnetron is circularly scanned around the target.
Sputter equipment is being currently commercialized for coating flat panel displays, for laptop computer screens and the like. Demaray et al. describes such a sputter chamber in U.S. Pat. No. 5,487,822. This PVD reactor is available from Applied Komatsu Technology, Inc. (AKT) of Santa Clara, Calif. The substrates are large rectangular pieces of glass having edges of up to 600 mm length, and larger sizes are envisioned for the future. The rectangular shape of the substrates has prompted the use of a linear array of permanent magnets for the magnetron with the array being linearly and reciprocally scanned in the direction perpendicular to the array axis. Halsey et al. describe such a linearly scanned magnetron in U.S. patent application Ser. No. 08/684,446, filed Jul. 19, 1996. Some have suggested using the same type of linear magnetron scanning in the circular geometry of sputter equipment intended for fabricating larger silicon wafers, for example, the 300 mm-wafer equipment now being developed.
The continuing development of increasingly complex integrated circuits, whether for memory or logic circuits, is based in large part on decreased minimum feature size, which is being pushed from a challenging 0.35 .mu.m for commercially available parts to 0.18 .mu.m and below for future generations now under development. The decreased feature size coupled with the increased number of features on a chip has exposed a problem with particles. Even inside a vacuum reactor, there tend to be significant number of particles, and the particle density increases with decreasing particle dimensions. A single small particle falling on an equally small feature can potentially cause the entire integrated circuit chip to fail. An immediate failure reduces manufacturing yield, sometimes to the vanishing point for a many-step fabrication process. A partial failure caused by a particle degrades performance. A particle embedded in a layer may not immediately cause failure, but may introduce a failure mode, e.g. localized heating as electrical current is forced around the insulating particle. that eventually produces a failure. That is, particles may cause long-term reliability problems. For these reasons, great efforts have been expended in all the technologies involved in fabricating integrated circuits to reduce the number of particles. The particle requirements, often in the range of less than one particle per wafer, are becoming very difficult to meet.
These problems extend to sputtering reactors even though they tend to operate at extremely low vacuums of 10.sup.-8 Torr and below.
Historically, particulate contamination has been controlled by strenuous cleaning methods and procedures for enforced cleanliness, e.g. clean rooms. clean room suits, and wafer cassettes. However, some estimate that greater than 80% of particles are generated by the processing equipment itself. As a result, further decreases in particle counts cannot rely only on conventional procedures.
Accordingly, it is greatly desired to develop new methods of reducing particles.
Bennett et al. in U.S. Pat. No. 5,367,139 has taught that many particles are formed from nucleation of the processing gas in plasma processing chambers and that these particles tend to become suspended at the plasma sheath. They believe the particles become negatively charged in their halide etching chemistry. They then suggest a number of methods of sweeping the particles away from the active processing area and trapping them away from the processing area, including modulating the RF power establishing the plasma. Praburam et al. describe the growth and suspension of particles within the plasma in "Observations of particle layers levitated in a radiofrequency sputtering plasma," Journal of Vacuum Science and Technology A, vol. 12, 1994 pp. 3137-3145. The positively charged plasma tends to trap negatively charged particles, particularly near the target where the electrostatic and gravitational forces balance. Praburam et al. describe how, once the plasma is extinguished, the particles fall to a surface in the chamber, particularly the wafer. Blanchard et al. in U.S. Pat. No. 5,221,425 also teach that particles can be suspended in the plasma of a magnetically enhanced reactive ion etcher. They suggest at the completion of etching while the wafer remains in the chamber to reduce the magnetic field and/or the RF plasma source bias so that the gas flow sweeps the suspended particles out of the chamber.
Sputtering relies upon the generation of a plasma within the sputtering chamber. A plasma is generated when the voltage applied across the processing gas exceeds the dielectric breakdown limit of the gas although sometimes a plasma igniter, such as a small arc or spark, is used to precipitate the breakdown. In any case, the ignition process is spatially non-uniform, similar to a lightening strike, and may dissipate large amounts of energy in localized areas of surfaces around the sputtering chamber. The energy may be sufficiently high to pit the surface, either vaporizing a small amount of material or dislodging solid material. In either case, particles are created. Once the plasma has been established, an equilibrium condition is established that is less prone to particle generation than is the ignition condition. As a result, the plasma ignition may be a major generator of particles. Some are entrained in the resultant plasma while others are immediately deposited on surrounding areas, including the wafer.